1. Field of the Invention
The present invention relates to a method of cooling heat releasing devices that must be cooled for their continuous and efficient functioning. More particularly, a method is provided for cooling by flooding electronics or other high-density power dissipation systems with a fine mist without directly impacting or wetting the system.
2. Description of the Prior Art
The removal of the heat load from electronic systems densely packed into ever-smaller spaces is a great challenge in thermal management, especially with the ever-increasing power of these systems. Other high-density power dissipation systems face similar challenges in heat source cooling applications. While the electronics industry strives to satisfy the increasing market demand for faster, smaller, lighter, and cheaper electronic devices through miniaturization, increasing power density in those devices tests the practical limits of traditional forced air convection cooling. Thus, the heat dissipation requirements of these electronics and high-density systems are rapidly becoming a limiting factor to their continued improvement.
Presently, thermal management of heat dissipation is accomplished by several cooling methods, both conventional and new, including natural air convection, forced air convection, conduction, heat-pipe technology, and radiation. Increasingly, these methods do not provide adequate cooling efficiency to maintain appropriate system temperatures under the extreme thermal environments being encountered. More recent research efforts have focused on dielectric liquid cooling, both by direct contact and spray technology. While more efficient in cooling power, these technologies have disadvantages associated with direct contact, sealed, wet systems.
U.S. Pat. No. 5,220,804 to Tilton, et al. discusses spray evaporative cooling of heat sources. Tilton teaches pressure atomizer nozzles arranged in arrays for deposition of a thin film of liquid onto a heat source. Upon phase change from liquid to gas, the film of liquid on the heat source cools the surface of electronics packed in an enclosure. Several references are provided in this patent (U.S. Pat. No. 5,220,804) regarding both spray and jet impingement cooling methods, wherein droplets of liquid cool the heat source surface by the contact and subsequent vaporization of the droplets. Tilton describes the direct impingement spray cooling method with a review of various parameters of importance such as spray angle, spray momentum, nozzle array geometry, uniform versus non-uniform cooling, targeted versus uniform spray, and liquid condensation. Isothermal System Research (ISR) appears to be pursuing this technology aggressively through SBIR grants from the U.S. NAVY. ISR has contributed significantly to commercial off the shelf (COTS) electronics cooling. ISR has utilized dielectric liquids as shown in Table 1 later. ISR has not been known by the present inventor to use water as coolant or to teach the fine-mist cooling method taught herein.
The spray cooling method developed by ISR appears to stem from U.S. Pat. No. 5,220,804. The ISR methodology uses direct impingement of liquid onto a heat source and has several limiting characteristics or disadvantages thereof. First, the ISR system must be highly engineered for successful implementation. For instance, the spray cooling system in the Tilton reference has a complex infrastructure including pressure spray nozzles and complex fluid handling systems, and the system has great potential for disadvantages including nozzle corrosion, erosion due to contaminants, and liquid recovery problems. The costs associated with these direct impingement systems make these systems less attractive. Consequently, new technology is still needed to address the thermal management needs of COTS electronics packages and prevent heat sources from causing catastrophic thermal failure.
U.S. Pat. No. 6,498,725 to Cole, et al. again teaches an evaporative spray cooling system using direct impingement of liquid on surfaces for removing high heat fluxes from surfaces of devices such as micro-electronic chips, metal, mirrors, and lasers. The system uses expanding two-phase flow and a method of controlling the spray for optimum heat flux removal. Controls include spray atomization, fluid-phase, mass flow, and spray temperature. A wide variety of spray atomizers exist. Cole teaches that for electronic cooling of chips with exposed leads or exposed electrical surfaces, the cooling fluid must have a high dielectric constant; otherwise the fluid may short circuit the chip causing it to fail.
Within the field of evaporative spray cooling using pressure-type atomizers, efforts have concentrated on manipulating conditions for improving deposition of the spray and the mechanics of impingement on the surface being cooled. Several atomization conditions have been explored and addressed including uniformity of spray, cooling surface coverage, momentum losses, and spray evaporation losses.
The authors of the present invention have not encountered work in patent literature or in the public domain discussing the merits of cooling electronics and/or heat dissipating systems using fine-mist flooding or circulating a fine mist within a system or electronics cabinet. The only reference found is to direct impingement spray cooling by water. According to a press release later presentation at a conference by University of California (UCLA) professors, directed spray jets were used to direct water droplets of about 35 micron diameter onto heat dissipating elements. In place of dielectric liquid spray disclosed by the above references, UCLA used pressurized water spray, while protecting electrical systems and the like with a coating. In the paper UCLA presented at the Eighth Intersociety Conference on Thermal and Thermo mechanical Phenomena in Electronic Systems, the scientists describe effective cooling by direct impingement spraying of water on high power density variable speed drive motors. The work reports effective thermal management using direct water sprays of high power devices in three-phase 18 hp AC motor drives. In order to protect the system from electrical shorting, the methodology used a coating of a copolymer, referred to as parylene, on sensitive surfaces.
The above work on direct impingement spray systems differs widely from the present invention in that spray cooling with water is simply a different direct impingement coolant from the dielectric liquid that was already taught by several inventors. Although the UCLA work shows more efficient cooling compared to dielectric liquids because of the high latent heat of vaporization, the technology is still complex, particularly because of the advanced coating required, requires a huge infrastructure to implement, and involves direct wetting of the heat source. Furthermore, the 35-micron droplets are still too large to absorb energy efficiently, and existing air-cooled systems cannot be refitted to use these technologies.
Thus, a cooling method for electronics and other heat dissipating systems is still needed to absorb and carry away large amounts of heat rapidly. The preferred system would take advantage of the extensive cooling design work already incorporated into most heat producing systems, would not require the extensive infrastructure associated with wetting systems, and would not cause physical degradation or breakdown. Preferably, the mist should be produced without pressure in order to avoid having a complicated fluid handling system with pumps and pressure lines.